Effect pigments produce an optical impression which depends on the angle of incidence and/or angle of observation of the applied coating. This is to be attributed on the one hand to the parallel-aligned orientation of the lamellar pigments within the application medium and on the other hand to the specific optical properties of the effect pigment. Thus, in particular, aluminum pigments act, on account of their high degree of reflection of visible light, like an ensemble of small mirrors. This manifests itself in a marked light-dark contrast (“brightness flop”) on observation from the specular angle up to more acute observation angles. Pigments of this type can be coated with various layers of variously refractive and/or colored materials. As a result, brilliant colored effect pigments are obtained. The coloring results in this case from a mixture of absorption, interference, and reflection phenomena. In particular, effect pigments having a steep color flop are obtained by utilizing interference phenomena.
Unlike pearl luster pigments, such effect pigments have very good covering power on account of their completely opaque aluminum core.
In addition to the silver-colored aluminum pigments, gold bronze pigments, which are alloys of copper and zinc, can produce golden shades. Further colored metal pigments can be produced by coating the same with colored and/or highly refractive oxides. Thus, iron oxide-coated aluminum pigments, which at the angle of incidence exhibit intensive golden to orange shades, are described in EP 33 457 and are obtainable under the trade name “Paliocrom®” supplied by BASF, Ludwigshafen, Germany. These are aluminum pigments coated only with iron oxide. The coloring here is produced by a mixture of the absorption color of the reddish iron oxide in the hematite modification, interference effects on the iron oxide layer (refractive index about 2.3), and reflection on the aluminum surface. At higher observation angles, however, these pigments only exhibit a color flop to uncolored without significantly changing their color location. In addition, the color range accessible is in practice restricted. The reason for this is the possiblity of the occurrence of the strongly exothermic thermite reaction:2 Al+Fe2O3→Al2O3+2 Fe
Since the iron oxide layer, as a coating, has the most intimate contact with the underlying aluminum pigment, this very exothermic reaction can be started by supply of a specific activation energy. On account of the finely divided nature of the pigments, “burnout” is associated with considerable safety risks. As a result, the iron oxide layer must in practice be restricted to a layer thickness which corresponds to a substoichiometric amount of iron oxide. In this way, however, strong red shades, for example, will not be accessible. On account of the interference spirals, such color shades would only appear at higher iron oxide layer thicknesses toward the color shades gold, orange and copper. These effect pigments are produced by CVD processes in a fluidized bed reactor. Iron oxide layers are precipitated onto the aluminum pigments fluidized in the fluidized bed by decomposition of iron pentacarbonyl in the presence of oxygen.
Furthermore, effect pigments are also known in which first a coating of a material having a low refractive index is applied and subsequently a partly transparent coating containing a highly refractive material is applied. The latter coating involves metal layers or highly refractive metal oxides or sulfides. In this case, interference pigments having a pronounced color flop, i.e. a change in the color location, are produced. Therefore, it is the desire with this class of interference pigments to produce homogeneous layers which are optically as perfect as possible.
The most spectacular effects with respect to color flops are achieved with effect pigments in which the coatings are vapor-deposited onto a thin metal substrate by means of PVD. Thus, according to the teachings of U.S. Pat. No. 5,059,245 and U.S. Pat. No. 5,135,812, pigments are first provided with a coating of a low-refracting material (n<1.65) and subsequently with a partially transparent metal coating. Optically highly homogeneous and uniform coatings are produced.
These pigments suffer from the drawback that on account of their manner of production they are not completely covered by the outer layers at their sides. If the metal core consists, or the outer layers consist, of aluminum, which is susceptible to corrosion, the use of these pigments in, for example, water-based paints would lead to gassing problems on account of the evolution of hydrogen. The outer metal layers can likewise cause corrosion problems. The extremely high production costs form a further disadvantage and also prevent the use of these pigments in many segments of the market.
EP 0 668 329 A2 describes an effect pigment in which aluminum pigments are first coated under chemical wet-process conditions with silicon oxide or hydrated oxide or with aluminum oxide or hydrated oxide. Subsequently, coating with metal by CVD processes or by currentless chemical wet-process metal deposition is carried out. Alternatively, non-selectively absorbing (colorless) metal oxides are deposited by a CVD processes in a fluidized bed reactor or alternatively under chemical wet-process conditions from organic metal compounds in an organic solvent.
In EP 0 708 154 A2, effect pigments are described having first a coating having a refractive index n<1.8 and subsequently a colored coating in which n>2.0. The highly refractive coating is preferably applied in a fluidized bed reactor by CVD processes, but chemical wet processes onvolving hydrolytic decomposition of organic metal compounds are also described in this reference. Strongly colored effect pigments are obtained, which exhibit an extremely steep color flop. When using iron oxide (Fe2O3) as the highly refractive layer, pigments in the reddish gold range are accessible.
These effect pigments, however, exhibit a number of disadvantages:
Thus, it is not always advantageous to apply layers which are as uniform and optically homogeneous as possible to the aluminum pigments. In this manner, the formation of intense interference colors and thus of steep color flops is indeed made possible and the color flops extend, in some cases, as far as to the complementary color range. However, steep color flops of this type do not provide real advantages in all applications. For instance, in the very large, stylistically rather conservative market segment of motor vehicle enamels, effect pigments with steep color flop are not desired, since the effect is regarded as being too intense by the customer. In the formulation of a motor vehicle enamel, in this case “flop breakers”, such as are described, for example, in EP 0 717 088, must be added in order to reduce the steep color flop effect. Such a solution, however, is uneconomical. Moreover, the designer can and will himself create color flops by suitably combining effect pigments with transparent or well-covering colored pigments. Effect pigments with a shallow color flop are therefore more preferable.
In addition, there is a need for metallic effect pigments with good covering power and colors not accessible hitherto such as red or green or copper colors which are weather-stable and exhibit no significant color flop. For example, there have hitherto been no red metal effect pigments or red metal effect pigment lacquers which do not simultaneously exhibit a blue tinge.
Generally, all CVD processes are more expensive than chemical wet-process coating processes. Chemical wet-process coating with SiO2 or aluminum oxides, however, has disadvantages: In order to attain the desired interference effects, certain layer thicknesses have necessarily to be achieved. In particular with silicon dioxide, on account of its lower refractive index (about 1.5), high layer thicknesses are necessary. As a result of this, however, the total thickness of the effect pigment becomes relatively large. This leads to poor covering power and relatively poor orientation behavior of the pigments and in some cases to spatial hindrance of the stacked pigments in the coating. As a result, losses of luster, luster haze phenomena and losses of the distinctness of image (DOI) occur in the coating.
In the direct chemical wet-process deposition of aluminum oxides or hydroxides on aluminum pigments further disadvantages occur: Layers of this type have only deficient gassing stabilities, since the deposited layers are not dense and are not homogeneous enough. Owing to their method of production, the aluminum pigments are coated with grinding aids such as fatty acids. These act as a sealing layer for the deposition of the aluminum oxide, which leads to layers which are pervious and which, moreover, do not adhere well to the substrate. Admittedly, according to the teaching of DE 42 23 384, the aluminum oxides can be largely freed from the fatty acids adsorbed on the surface in a vapor-phase reaction by reaction with water in an oxygen-containing turbulent atmosphere. However, such a procedure is not economical on account of the high costs.
Exclusive chemical wet-process covering of aluminum pigments with oxide layer packages is described in WO 00/09617. Here, all coatings are carried out in aqueous medium. Different layers are created at different, fixed pHs. First, an amorphous glassy layer (SiO2, phosphate, borate) is applied and subsequently a highly refractive oxide is deposited.
Since the oxide coating is carried out at, in some cases, extreme pHs, the aluminum pigments must be passivated beforehand. Possibilities mentioned in this case are treatment with hydrogen peroxide or nitric acid. This process does not, however, produce well-passivated pigments. Moreover If the treatment is too vigorous, the optical properties (brilliance, brightness) of the pigments are markedly impaired. Thus the problem arises of inadequate gassing stability during the subsequent oxide coating operations and in the final product.
A great disadvantage of all multilayer pigments known in the prior art which are prepared under chemical wet-process conditions or by CVD deposition is the fact that all layers must be deposited on the aluminum pigment as a starting substrate. The aluminum pigment is thus present as a reflector core in its original thickness even in the interference pigment. This thickness, however, due to the method of production, is far greater than that layer thickness which would be necessary for optical impermeability and thus for achieving very good covering power. Therefore, the aluminum cores in the pigments disclosed in the aforementioned specifications are thicker than necessary. This is manifested by a loss of covering power. All further coatings, moreover, increase the total layer thickness of the effect pigment, which leads to decreased luster, luster haze problems in the coating and poor distinctness of image.
In EP 0 848 735, aluminum pigments oxidized under chemical wet-process conditions are described which are oxidized in a mixture of water and an organic solvent. The pigments do not have to be subjected to any degreasing treatment at all prior to oxidation. The pigments exhibit colors ranging from nickel to bright gold to bronze. These colors, however, are only weak and very limited in their color range.
In German Laid-open Specification DE 26 27 428, there is described a process for the production of colored aluminum powder. The deposition of a metal salt and an organic chelating agent from a weakly alkaline solution is disclosed. The aluminum pigments can in this case be subjected in a first treatment stage to passivation with a superficial smooth boehmite film. In addition, the superficial and smooth boehmite film produced according to the teaching of DE 26 27 428 is restricted as to layer thickness and lies far below the thickness range starting from which this layer would be effective as an interference zone for the production of interference colors.
Using the process according to the teaching of DE 26 27 428, colored pigments are obtained which only lie in the gold range. Red or green pigments are not obtainable thereby.